WASHINGTON -- Chimpanzees may have been using stone "hammers" as long as 4,300 years ago. An international research team, led by archaeologist Julio Mercader of the University of Calgary, Canada, said Monday it had uncovered the hammers, dated to that time, in the West African country Ivory Coast. It would be the earliest known use of tools by chimpanzees.

The hammers were used to crack nuts, a behavior still seen in chimps in that area, the researchers said in a paper in the online edition of Proceedings of the National Academy of Sciences.

The finding may indicate that a "chimpanzee stone age" began in ancient times, the researchers say.

The earliest reports of stone tool use by chimpanzees in this area date to the writings of Portuguese explorers in the 1600s.

The stones were about the size of cantaloupes with patterns of wear indicating use to crack nuts. The rocks would have been too large for human hands, but about right for the larger, stronger hands of chimpanzees, the researchers said.

"It's not clear whether we hominins invented this kind of stone technology, or whether both humans and the great apes inherited it from a common forebear," Mercader said in a statement.

But, he added, there weren't any farmers living in this region 4,300 years ago, so it is unlikely chimpanzees picked it up by imitating villagers.

Others aren't so sure.

The tools may predate farming in the area but not necessarily human contact, said anthropologist Stanley H. Ambrose of the University of Illinois at Urbana-Champaign. There were early human hunter-gatherers in the region at the time, he said.

If chimps and early humans shared this technology with a common ancestor between 5 million and 7 million years ago, there should be sites with chimp debris going back 5 million years and the earliest human stone tool sites should show the kinds of debris found at the chimp sites, Ambrose said: "They absolutely do not."

Some of the hammers also had starch residue on them, mostly from types of nuts that are still eaten by chimpanzees, but not humans. Some of the starch was also associated with tubers but the researchers interpreted that as "background noise" probably picked up in the ground.

Skeptical of the starch findings was anthropologist Jeanne Sept, dean of faculties at Indiana University.

"Their interpretations of the starch grains recovered from the specimens are incomplete, and somewhat circular," she said. "For example, they do not describe which surfaces of the rock fragments the starch grains were obtained from, which makes it difficult to judge whether the grains were left on the tools as a result of tool-use, or if the grains merely naturally stuck to the stones after they became buried in the soil."

"They assert that starch grains from nuts were behaviorally significant, while they decide that starch grains identified as yam roots are 'background noise' but give no justification for that," she added. "They may in fact be dismissing or ignoring evidence that does not match their chimp-nut interpretation of the site."

"It may be premature to accept the range of their claims until further evidence is presented," Sept concluded.

Mercader's research was funded primarily by the Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany; Canada Research Chairs program, Canada Foundation for Innovation and the University of Calgary.